Studies on the changes of mitochondrial form in both yeast cells and in a human leukemia cell line, HL60 are being carried out in an attempt to understand the plastic nature of this cellular organelle. The occurrence of cells with a few giant mitochondria in the same population of cells that contain many small mitochondria suggests that some cellular process controls their configuration. The discovery that the mitochondrial DNA (mtDNA) can be visualized by fluorescence microscopy in certain yeast resulted in the finding that the mtDNA often assumes a network-like configuration in some fraction of cells in the population. Together these findings imply that mitochondrial form and its organellar DNA follow similar pattern changes. Studies on the mtDNA of a yeast, Wickerhamia fluorescens show that it has a repeat length of 21 micra and that the microscopically observed elements must be made of an association of these units. The nature of this association is being investigated. Mitochondrial DNA is not observable by fluorescence microscopy in higher cells (HL60) but mitochondria in various sizes can be readily observed in these cells by specific stains. How these mitochondrial shapes are correlated with the cell cycle age, culture age or stage in differentiation is the subject of this investigation. Work on how the mitochondrial mass changes over the cell cycle in HL60 cells is being carried out using flow-microfluorimetry. The fluorescent dye, rodamine 123, has proved to be highly specific in this study.